1. Field of the Invention
Embodiments of the present invention relate generally to compositions and methods designed to aid in the treatment or detection of asthma or allergies.
2. Description of the Related Art
Asthma, a chronic disorder which causes detrimental, and in some cases, potentially fatal pulmonary inflammation affects 15 million Americans and accounts for approximately 12.7 billion dollars in health care costs each year. Despite extensive ongoing research, asthma is currently on the rise. The inability of researchers to develop an effective treatment for asthma is largely due to the complexity of the disease. Discovering effective treatments with broad applicability is extremely difficult because asthma derives from a wide number of factors. For example, multiple specific inflammatory pathways, many of which are poorly understood, are thought to interplay with one another to produce the symptoms that result in a diagnosis of asthma in a patient. In addition, research is further complicated by the fact that the relative importance of those pathways can differ between individual asthma sufferers.
Experimentation in the asthma field has largely focused on analysis of the cellular and molecular events induced by allergen exposure in sensitized animals (primarily mice) and humans. These studies have identified elevated production of IgE, mucus hypersecretion, airways obstruction, inflammation and enhanced bronchial reactivity to spasmogens in the asthmatic response. Clinical and experimental investigations have demonstrated a strong correlation between the presence of CD4+T helper 2 lymphocytes (Th2 cells) and disease severity suggesting an integral role for these cells in the pathophysiology of asthma. Th2 cells are thought to induce asthma through the secretion of an array of cytokines that activate inflammatory and residential effector pathways both directly and indirectly. In particular, interleukin-4 (IL-4) and interleukin-13 (IL-13) are produced at elevated levels in the asthmatic lung and are thought to be central regulators of many of the hallmark features of disease.
Arginine Metabolism
L-arginine is a semi-essential basic amino acid that is involved in two biochemical pathways, the citrulline-nitric oxide (NO) cycle and the urea cycle as illustrated in FIG. 1. The bulk of the urea cycle occurs in the liver, the main organ containing the full enzymatic machinery necessary for the urea cycle. The enzyme arginase is the only urea cycle enzyme that exists in two isoforms (60% amino acid homology), which are encoded by different genes on distinct chromosomes, designated type I and type II. Arginase I is a cytoplasmic protein that is primarily expressed in the liver; whereas arginase II is a mitochondrial protein expressed in a variety of tissues, especially the kidney and prostate. The downstream enzymes ornithine decarboxylase (ODC) and L-ornithine amino transferase (OAT) are specifically expressed in the cytoplasm and mitochondria, respectively, suggesting coordinated biochemical links for the two isoenzymes.
Arginase I deficiency in humans results in hyperargininemia and a progressive neurological deterioration that is usually fatal. Whereas arginase I deficient transgenic mice die within 9-11 days after birth, arginase II deficient mice are grossly normal. One development in the past several years concerning L-arginine metabolism was the finding that arginase can be expressed in many tissues and cell types following exposure to a variety of cytokines and agents. Of the cytokines shown to regulate arginase, IL-4, IL-10, and IL-13 appear to be the most potent, especially in macrophages. Although both arginases are inducible by various stimuli in vitro, arginase I appears to be more strongly induced by Th2 cytokines. However, this has not been extensively studied in cell types other than macrophages.
The exact function of arginase in extrahepatic tissue is not well understood. However, the product of arginase, L-ornithine, is a precursor in the production of polyamines (e.g. putrescine, spermidine, and spermine) and proline, which control cell proliferation and collagen production, respectively. In fact, increased expression of arginase I alone is sufficient to result in increased proliferation rates of vascular smooth muscle and endothelial cells. Thus, arginase activity is potentially critically linked to cell growth and connective tissue production, notably, both of these processes are hallmark pathological features of chronic asthma and allergies (FIG. 1).
In addition to being metabolized to L-ornithine, L-arginine is also a precursor of NO, a free radical molecule involved in a wide range of biological processes. NO is formed from L-arginine by the enzyme NOS. Three isoforms of NOS have been described. NOS1 and NOS3 are constitutively expressed and their activity is calcium dependent. NOS1 is expressed in neurons and is thought to have a role in neurotransmission, whereas NOS3, or endothelial NOS, has a role in smooth muscle relaxation and bronchodilation. NOS2, inducible NOS (iNOS), is calcium-independent, and is up-regulated in response to inflammatory mediators such as endotoxin and interferon-γ, leading to the production of large amounts of NO.
The diagram in FIG. 1 illustrates the role of cationic amino acid transporter-2 (CAT2) in the arginase pathway. Extracellular L-arginine is required for sustained NO and L-ornithine generation from L-arginine, implicating an important role for L-arginine transport through the plasma membrane. Among the several transport systems that mediate L-arginine uptake, system y+is widely expressed and considered the major L-arginine transporter in most cells and tissues. Encoded by cationic amino acid transporters CAT1, CAT2, and CAT3, system y+is a Na+-independent high affinity cationic amino acid transport system. With the exception of the liver, CAT1 is expressed virtually ubiquitously and is required for viability, whereas CAT2 is expressed in a more restricted number of tissues; CAT3 is primarily expressed in the brain.
Due to differential splicing of two exons, CAT2 mRNA exists in two isoforms: CAT2A, a low affinity transporter that is expressed primarily in the liver, and the high affinity CAT2 (CAT2B). CAT1 and -2 are homologous proteins that lack a signal peptide but contain 12 transmembrane spanning domains with an intracellular amino-terminus. Interestingly, CAT2 was originally cloned from lymphoma cell line cDNA and was named Tea (T cell early activation factor), because it is induced early in the response of normal T cells to mitogens. However, the role of CAT2 in T cell immune responses has not yet been reported but preliminary studies have indicated an important role for this molecule in experimental autoimmune encephalitis. The first indication that CAT2 may be involved in critically regulating substrate availability for iNOS or arginase was the finding that pro-inflammatory molecules (e.g. lipopolysaccharide [LPS]) regulate CAT2 expression. In contrast, cat-1 is a “housekeeping” gene that is not induced under conditions that induce CAT2. A further interesting relationship has been established by the finding that eosinophil cationic proteins inhibit L-arginine uptake by macrophages. Recent analysis of CAT2-deficient mice has revealed that sustained NO production in macrophages requires CAT2. The 95% decrease in L-arginine uptake by CAT2 deficient macrophages, indicates that CAT2 is the major L-arginine transporter in macrophages.
CAT2 was originally cloned from lymphoma cell line cDNA and was named Tea (T cell early activation factor), because it was induced early in the response of normal T cells to mitogens. (MacLeod et al., J Exp Biol, 196:109-21 (1994)). However, previous studies on the role of CAT2 in immune responses have been primarily limited to its effects on NO production (Nicholson et al., J Biol Chem, 276:15881-5 (2001)). It was thus important to further characterize exactly which CAT2 isoform is expressed in the asthmatic lung. CAT2 is expressed as two separate isoforms depending upon the specific utilization of exon 7 (Type 2B) or exon 8 (Type 2A) (Nicholson et al., J Biol Chem, 276:15881-5 (2001)). CAT2A has a lower affinity for L-arginine and is thought to be mainly expressed in the liver (MacLeod et al., J Exp Biol, 196:109-21 (1994)).
Because incidence of asthma and allergies are on the rise, research leading to a better understanding and treatment of this disease is needed. Thus, what is needed in the art are new methods of treating an individual suffering from asthma or allergies, new methods for detecting individuals at risk for asthma or allergies, and new methods for phenotyping patients (e.g. predicting their prognosis and response to treatment).